This invention relates to production of high melt flow propylene polymer and, particularly, relates to producing high melt flow polypropylene by gas-phase polymerization without additional peroxide treatment.
Recently, there has been widespread use of solid, hydrocarbon-insoluble, magnesium halide-supported, titanium-containing high activity catalysts ("HAC") to produce propylene polymer resins. Use of solid, transition metal-based, HAC, olefin polymerization catalyst components is well known in the art including such solid components supported on a metal oxide, halide or other salt such as widely-described magnesium-containing, titanium halide-based catalyst components.
In addition to the solid, magnesium-containing, titanium-containing HAC catalyst component, the polymerization catalyst system used to produce propylene polymers uses an aluminum alkyl component, such as triethylaluminum, and typically an external modifier component such as a silane compound as described in U.S. Pat. No. 4,829,038, incorporated by reference herein. Use of external silane modifiers in a propylene polymerization catalyst system has been widely described. Use of alkyl or aryl methoxysilanes, and particularly dialkyldimethoxysilanes, has been described. The present invention describes using a specific silane composition as an external modifier together with specific process conditions to produce a high melt flow polymer in a gas-phase polymerization reactor, which has not been possible in conventional process systems.
Melt flow rate (MFR) is measured in units of grams of polymer extruded under standard conditions per unit of time, usually grams/10 minutes according to ASTM D1238 Condition L. For high MFR, ASTM 1238 230/2.16 with a die diameter of 1.045 mm and length of 4.0 mm may be used. A standard calibration factor is used to correlate to ASTM D1238 measurements. Typically, MFR is controlled in a propylene polymerization reactor by varying the hydrogen content in the reactor. However, in order to produce high melt flow rate polymer, increased hydrogen concentration leads to a loss of catalyst activity to an extent which makes direct production of high melt flow polymer commercially impractical. In bulk or slurry polymerization systems, the melt flow rate of a polymer is limited by the effective solubility of hydrogen in the liquid polymerization medium in combination with pressure limitations on the polymerization reactor system.
Conventionally, propylene polymers having a melt flow rate above about 400 g/10 min. are manufactured by a secondary treatment of a reactor-produced polymer powder with a peroxide using controlled rheology techniques. Although controlled rheology peroxide treatment produces high melt flow products with melt flow rates up to 3000 g/10 min. and above, a controlled rheology process requires additional manufacturing equipment and is less efficient than a process which could produce high melt flow product directly in a polymerization reactor. Further, products produced through controlled rheology methods, contain oxygenated residues and very short chain oligomers which may cause fiber breaks if the polymer is used to manufacture melt blown fiber. A reactor-produced high melt flow product is more uniform than a controlled rheology product, which leads to better processing for melt blown fiber production.
This invention describes a process to produce high melt flow polypropylene directly in a gas-phase reactor at acceptable commercial yields without use of a secondary controlled rheology treatment. This process is especially useful in a gas-phase process in which gaseous monomer is condensed into a liquid which may be used to regulate reaction temperature. Since any hydrogen used for melt flow rate control cannot be condensed in such a process, there is a practical maximum to hydrogen concentration. Such hydrogen concentration maximum limits the ability of such process to produce high melt flow product directly in the reactor.
In this invention, a gas-phase polymerization process, typically using a supported catalyst system and an aluminum alkyl co-catalyst, incorporates tetraethylorthosilicate (TEOS) as an external catalyst modifier under controlled process conditions in the presence of hydrogen to produce high melt flow polypropylene directly in the polymerization reactor.
Tetraethylorthosilicate (also known as tetraethoxysilane) has been used in propylene polymerization catalyst systems. For example, in an early generation catalyst system using a ball-milled co-crystallized titanium trichloride/aluminum trichloride catalyst component, TEOS was used commercially as a co-catalyst modifier with ethyl aluminum dichloride.
Published application EP 0 445 303 describes an olefin polymerization catalyst composed of a solid catalytic component containing a metal oxide, magnesium, titanium, halogen, and an electron-donating compound, (b) an organometallic compound, and (c) an ethoxysilane compound with a formula R.sub.n Si(OC.sub.2 H.sub.5).sub.4-n, wherein R is a C.sub.3 to C.sub.10 aliphatic compound and n is 1 or 2. The catalyst system is described as good in the effect of hydrogen on the melt flow rate. However, the exemplified polymerization process was in slurry and the disclosed polymers had melt flow rates below 400 g/10 min.
Published application EP 0 601 496 describes an olefin polymerization process in which two polymerizations are allowed to coexist. In one polymerization catalyst system, a dialkylalkoxysilane is used as an external modifier, which in the second system an alkoxysilane is used such as propyltriethoxysilane or vinyltriethoxysilane.
Published application WO 95/21203 describes a dual donor catalyst system which includes a mixture of dicyclopentyldimethoxysilane and TEOS. The polymers produced are described a having relatively high melt flow rates and moderately broad molecular weight distributions. However, the exemplified polymerization process is bulk and the highest melt flow polypropylene described had an MFR under 250 g/10 min. Also, shown was that catalyst activity dropped substantially when higher melt flow rate polymers were made.
Published application WO 94/06833 describes the use of TEOS as an external modifier to a specific supported catalyst system in bulk and fluidized bed gas-phase polymerization systems but does not disclose high melt flow polymers.
U.S. Pat. No. 5,529,850 describes a fiber produced from a polyproylene resin using 2,2,6,6-tetramethyl piperidine-containing external modifier and an ether internal electron donor. The resin is characterized by a polydispersity index of 2.5 to 3.7.
An aspect of this invention is to provide a high melt flow propylene polymer. Another aspect of this invention is a process to manufacture high melt flow polypropylene in a polymerization reactor. A further aspect of this invention is a process to manufacture reactor-produced high melt flow propylene polymers at commercially-acceptable yields based on a titanium-containing catalyst component.
In another aspect of this invention, the use of a non-silane external modifier to increase melt flow rate is significantly enhanced. Use of diethyl zinc (DEZ) as an external modifier is known to increase melt flow rate of propylene polymers. However, at the concentration of DEZ required for DEZ to form high MFR polymer, it has been found that the polymer is contaminated with black specks. Use of TEOS in combination with DEZ permits high melt flow rate polymer to be produced at lower concentrations of DEZ without formation of black specks.
The high melt flow products of this invention are useful in producing melt blown fibers, which, typically, may form a non-woven fabric. Uses for melt blown fibers include filters, oil sorbents, and as components in composite non-woven fabrics for medical and hygiene uses.